One aspect of the current invention is an optimized high yield nucleic acid delivery vehicle, or synthetic expression plasmid. The synthetic expression plasmid of this invention has reduced components, and has been optimized to increase yield. In addition to a mammalian gene of interest, a typical nucleic acid delivery vehicle or synthetic expression plasmid contains many structural elements necessary for the in vitro amplification of the plasmid in a bacterial host. By restricting the plasmid backbone to essential bacterial structural elements (e.g. bacterial antibiotic resistance gene and origin of replication) one can eliminate detrimental sequences, but not affect the final gene product. By introducing targeted substitutions in the bacterial origin of replication, one can increase plasmid yield and decrease fermentation time, thus increasing productivity. The current invention involves a “synthetic plasmid backbone” that provides a small backbone with an improved origin of replication (“mut 8”), which is useful for plasmid supplementation therapy in mammals.
A plasmid based mammalian expression system is minimally composed of a plasmid backbone, and a nucleic acid sequence encoding a therapeutic expression product under the transcriptional regulation of a promoter, and followed by a 3′UTR and/or polyadenylation signal. A plasmid backbone typically contains elements necessary for the specific growth of only the bacteria that are transformed with the proper plasmid: (1) a bacterial origin of replication, and (2) a selection marker, typically a bacterial antibiotic resistance gene. However, there are plasmids, called mini-circles, that lack both the antibiotic resistance gene and the origin of replication (Darquet et al., 1997; Darquet et al., 1999; Soubrier et al., 1999). Production and purification of mini-circles is complex and extremely costly, and thus impractical for therapeutic applications in large animals and humans.
The use of in vitro amplified expression plasmid DNA (i.e. non-viral expression systems or plasmids) avoids the risks associated with viral vectors (Frederickson et al., 2003). The non-viral expression systems products generally have low toxicity due to the use of “species-specific” components, or components that are not expressed in eukaryotic cells, which minimizes the risks of immunogenicity generally associated with viral vectors. One aspect of the current invention is a new, versatile, optimized high yield plasmid-based mammalian expression system that will reduce the risk of adverse effects associated with prokaryotic nucleic acid sequences in mammalian hosts, while facilitating its in vitro production. In addition, this new plasmid will constitute the base of a species-specific library of plasmids for expression of hormones or other proteins for agricultural, companion animal and human applications.
The nucleotide sequence of the bacterial gene products can adversely affect a mammalian host receiving plasmid DNA. For example, it is desirable to avoid CpG sequences, as these sequences have been shown to cause a recipient host to have an immune response targeted against the plasmids (Manders and Thomas, 2000; Scheule, 2000), as well as possible gene silencing (Shi et al., 2002; Shiraishi et al., 2002). Thus, when properly designing and generating DNA coding regions of any expressed genes one could avoid the “cg” sequence, without changing the amino acid sequence. This process, called “optimization” was used to generate the plasmids described in the present application. Another aspect of the current invention involves the removal of unnecessary backbone DNA sequences that were shown to have no functionality in the new plasmid context. As a result of removal of unnecessary DNA sequences, a new plasmid backbone (“pAV9001”) with unique cloning sites was constructed, which will be useful for plasmid-mediated gene supplementation.
RNA II is the primer for replication of ColE1-derived plasmids and it is inhibited by RNA I (Dasgupta et al., 1987; Gayle, III et al., 1986). Increasing the RNA II to RNA I ratio should increase the frequency of DNA replication initiation events, which should yield higher plasmid copy number. The danger is the production of levels of RNA II that are so elevated that they would lead to “runaway” plasmid replication. Thus, strict control of the relative potency of these sequences is necessary. In another embodiment of this invention, new improved RNA II sites were created. Plasmids containing these new sequences have decreased fermentation time to optimum concentration and result in high plasmid yields.
Two conserved regions about 35 and 10 base pairs (bp) upstream from the transcription start (−35 and −10 regions, respectively) were identified by comparison of numerous promoters (Harley and Reynolds, 1987). Extensive compilations and comparison of promoters of genes of E. coli and it plasmids supported and extended the concept of a “consensus” promoter sequence: a −35 (TTGACA) and −10 (TATAAT) region separated by 17 bp with transcription initiation at a purine about 7 bp downstream from the 3′end of the −10 region (Ross et al., 1998). While the −35 and −10 regions show the greatest conservation across promoters and are also the sites of nearly all mutations which affect transcriptional strength, other bases flanking the −35 and −10 regions, in addition to the start point also occur at greater than random frequencies and sometimes affect promoter activity (Bujard et al., 1983; Deuschle et al., 1986; Kammerer et al., 1986). In addition, variations in spacing between the −35 and −10 regions play a role in promoter strength.
Point mutations in the RNA II promoter: A mutation described in (Bert et al., 2000), alters the −10 element in the RNA II promoter from TAATCT to TAATAT in a ColE1-derived plasmid named pXPM. The mutated sequence is a closer match to the consensus −10 element described above, and was therefore predicted to increase the rate of RNA II transcription. Nevertheless, the effect of mutations in the particular context of each plasmid is highly unpredictable. We modified a plasmid with a pUC origin of replication which requires special fermentation condition, and already contains a mutation of a C to a T (located at position +112 of RNA II in the region of stem/loop IV that hybridizes to 5′ end of RNA I) that increases plasmid copy number (Lahijani et al., 1996; Lin-Chao et al., 1992). The effectiveness and results of combination of mutations is unpredictable, and can be assessed only by measuring plasmid yield, after the sequence has been synthetically generated.
There are several different approaches that can be utilized for the treatment of chronic conditions, such as cancer, arthritis, renal failure or immune dysfunction. Effective treatment may require the presence of therapeutic agents for extended periods of time. In the case of proteins, this is problematic. Gene therapeutic approaches may offer a solution to this problem. Experimental studies have confirmed the feasibility, efficacy and safety of plasmid-mediated gene supplementation for the treatment of chronic conditions.
Direct plasmid DNA gene transfer is currently the basis of many emerging nucleic acid therapy strategies and does not require viral components or lipid particles (Aihara and Miyazaki, 1998; Muramatsu et al., 2001). Skeletal muscle is target tissue, because muscle fiber has a long life span and can be transduced by circular DNA plasmids that are expressed in immunocompetent hosts (Davis et al., 1993; Tripathy et al., 1996). Plasmid DNA constructs are attractive candidates for direct therapy into the subjects skeletal muscle because the constructs are well-defined entities that are biochemically stable and have been used successfully for many years (Acsadi et al., 1991; Wolff et al., 1990). The relatively low expression levels of an encoded product that are achieved after direct plasmid DNA injection are sometimes sufficient to indicate bio-activity of secreted peptides (Danko and Wolff, 1994; Tsurumi et al., 1996). Previous reports demonstrated that human GHRH cDNA could be delivered to muscle by an injectable myogenic expression vector in mice where it transiently stimulated GH secretion to a modest extent over a period of two weeks (Draghia-Akli et al., 1997).
Efforts have been made to enhance the delivery of plasmid DNA to cells by physical means including electroporation, sonoporation, and pressure. Although not wanting to be bound by theory, the administration of a nucleic acid construct by electroporation involves the application of a pulsed electric field to create transient pores in the cellular membrane without causing permanent damage to the cell, which allows exogenous molecules to enter the cell (Smith and Nordstrom, 2000). By adjusting the electrical pulse generated by an electroporetic system, nucleic acid molecules can travel through passageways or pores in the cell that are created during the procedure. U.S. Pat. No. 5,704,908 titled “Electroporation and iontophoresis catheter with porous balloon,” issued on Jan. 6, 1998 with Hofmann et al., listed as inventors describes an electroporation apparatus for delivering molecules to cells at a selected location within a cavity in the body of a patient. Similar pulse voltage injection devices are also described in: U.S. Pat. No. 5,702,359 titled “Needle electrodes for mediated delivery of drugs and genes,” issued on Dec. 30, 1997, with Hofmann, et al., listed as inventors; U.S. Pat. No. 5,439,440 titled “Electroporation system with voltage control feedback for clinical applications,” issued on Aug. 8, 1995 with Hofmann listed as inventor; PCT application WO/96/12520 titled “Electroporetic Gene and Drug Therapy by Induced Electric Fields,” published on May 5, 1996 with Hofmann et al., listed as inventors; PCT application WO/96/12006 titled “Flow Through Electroporation Apparatus and Method,” published on Apr. 25, 1996 with Hofmann et al., listed as inventors; PCT application WO/95/19805 titled “Electroporation and Iontophoresis Apparatus and Method For insertion of Drugs and genes into Cells,” published on Jul. 27, 1995 with Hofmann listed as inventor; and PCT application WO/97/07826 titled “In Vivo Electroporation of Cells,” published on Mar. 6, 1997, with Nicolau et al., listed as inventors, the entire content of each of the above listed references is hereby incorporated by reference.
Recently, significant progress to enhance plasmid delivery in vivo and subsequently to achieve physiological levels of a secreted protein was obtained using the electroporation technique. Electroporation has been used very successfully to transfect tumor cells after injection of plasmid (Lucas et al., 2002; Matsubara et al., 2001)) or to deliver the anti-tumor drug bleomycin to cutaneous and subcutaneous tumors in humans (Gehl et al., 1998; Heller et al., 1996). Electroporation also has been extensively used in mice (Lesbordes et al., 2002; Lucas et al., 2001; Vilquin et al., 2001), rats (Terada et al., 2001; Yasui et al., 2001) and dogs (Fewell et al., 2001) to deliver therapeutic genes that encode for a variety of hormones, cytokines or enzymes. Previous studies using GHRH showed that plasmid therapy with electroporation is scalable and represents a promising approach to induce production and regulated secretion of proteins in large animals and humans (Draghia-Akli et al., 1999; Draghia-Akli et al., 2002c). Electroporation also has been extensively used in rodents and other small animals (Bettan et al., 2000; Yin and Tang, 2001). Intramuscular injection of plasmid followed by electroporation has been used successfully in ruminants for vaccination purposes (Babiuk et al., 2003; Tollefsen et al., 2003). It has been observed that the electrode configuration affects the electric field distribution, and subsequent results (Gehl et al., 1999; Miklavcic et al., 1998). Although not wanting to be bound by theory, needle electrodes give consistently better results than external caliper electrodes in a large animal model.
The ability of electroporation to enhance plasmid uptake into the skeletal muscle has been well documented. Similarly, plasmids formulated with poly-L-glutamate (“PLG”) or polyvinylpyrrolidone (“PVP”) were observed to have an increase in plasmid transfection, which consequently increased the expression of a desired transgene. For example, plasmids formulated with PLG or PVP were observed to increase gene expression to up to 10 fold in the skeletal muscle of mice, rats, and dogs (Fewell et al., 2001; Mumper et al., 1998). Although not wanting to be bound by theory, the anionic polymer sodium PLG enhances plasmid uptake at low plasmid concentrations and reduces any possible tissue damage caused by the procedure. PLG is a stable compound and it is resistant to relatively high temperatures (Dolnik et al., 1993). PLG has been used to increase stability of anti-cancer drugs (Li et al., 2000) and as “glue” to close wounds or to prevent bleeding from tissues during wound and tissue repair (Otani et al., 1996; Otani et al., 1998). PLG has been used to increase stability in vaccine preparations (Matsuo et al., 1994) without increasing their immunogenicity. PLG also has been used as an anti-toxin after antigen inhalation or exposure to ozone (Fryer and Jacoby, 1993).
Although not wanting to be bound by theory, PLG increases the transfection of the plasmid during the electroporation process, not only by stabilizing the plasmid DNA and facilitating the intracellular transport through the membrane pores, but also through an active mechanism. For example, positively charged surface proteins on the cells could complex the negatively charged PLG linked to plasmid DNA through protein-protein interactions. When an electric field is applied, the surface proteins reverse direction and actively internalize the DNA molecules, a process that substantially increases the transfection efficiency. Furthermore, PLG will prevent the muscle damage associated with in vivo plasmid delivery (Draghia-Akli et al., 2002b) and will increase plasmid stability in vitro prior to injection. There are studies directed to electroporation of eukaryotic cells with linear DNA (McNally et al., 1988; Neumann et al., 1982) (Toneguzzo et al., 1988) (Aratani et al., 1992; Naim et al., 1993; Xie and Tsong, 1993; Yorifuji and Mikawa, 1990), but these examples illustrate transfection into cell suspensions, cell cultures, and the like, and such transfected cells are not present in a somatic tissue.
U.S. Pat. No. 4,956,288 is directed to methods for preparing recombinant host cells containing high copy number of a foreign DNA by electroporating a population of cells in the presence of the foreign DNA, culturing the cells, and killing the cells having a low copy number of the foreign DNA.
Although not wanting to be bound by theory, a GHRH cDNA can be delivered to muscle of mice and humans by an injectable myogenic expression vector where it can transiently stimulate GH secretion over a period of two weeks (Draghia-Akli et al., 1997). This injectable vector system was optimized by incorporating a powerful synthetic muscle promoter (Li et al., 1999) coupled with a novel protease-resistant GHRH molecule with a substantially longer half-life and greater GH secretory activity (pSP-HV-GHRH) (Draghia-Akli et al., 1999). Highly efficient electroporation technology was optimized to deliver the nucleic acid construct to the skeletal muscle of an animal (Draghia-Akli et al., 2002b). Using a skillful combination of vector design and electric pulses plasmid delivery method, the inventors were able to show increased growth and favorably modified body composition in pigs (Draghia-Akli et al., 1999; Draghia-Akli et al., 2003) and rodents (Draghia-Akli et al., 2002c), and improved immune surveillance in dairy cattle (Brown et al., 2004). The modified GHRH nucleic acid constructs increased red blood cell production in companion animals with cancer and cancer treatment-associated anemia (Draghia-Akli et al., 2002a).
Administering novel GHRH analog proteins (U.S. Pat. Nos. 5,847,066; 5,846,936; 5,792,747; 5,776,901; 5,696,089; 5,486,505; 5,137,872; 5,084,442, 5,036,045; 5,023,322; 4,839,344; 4,410,512, RE33,699) or synthetic or naturally occurring peptide fragments of GHRH (U.S. Pat. Nos. 4,833,166; 4,228,158; 4,228,156; 4,226,857; 4,224,316; 4,223,021; 4,223,020; 4,223,019) for the purpose of increasing release of growth hormone have been reported. A GHRH analog containing the following mutations have been reported (U.S. Pat. No. 5,846,936): Tyr at position 1 to His; Ala at position 2 to Val, Leu, or others; Asn at position 8 to Gln, Ser, or Thr; Gly at position 15 to Ala or Leu; Met at position 27 to Nle or Leu; and Ser at position 28 to Asn. The GHRH analog is the subject of U.S. Pat. No. 6,551,996 titled “Super-active porcine growth hormone releasing hormone analog,” issued on Apr. 22, 2003 with Schwartz, et al., listed as inventors (“the '996 patent”), which teaches application of a GHRH analog containing mutations that improve the ability to elicit the release of growth hormone. In addition, the '996 patent application relates to the treatment of growth deficiencies; the improvement of growth performance; the stimulation of production of growth hormone in an animal at a greater level than that associated with normal growth; and the enhancement of growth utilizing the administration of growth hormone releasing hormone analog and is herein incorporated by reference.
In summary, prior art has shown that it is possible to create new plasmids in a limited capacity utilizing existent plasmids or fragments from previously produced plasmids, but these techniques, and the resultant plasmids have some significant drawbacks: numerous CpG islands that can inhibit and reduce expression after treatment in vivo, large plasmid backbones that can accommodate relatively small transgenes, numerous bacterial elements with unknown function in eukaryotic cells, multiple cloning site regions with unknown effect on expression and replication, etc. It has also been taught that nucleic acid expression plasmids that encode recombinant proteins are viable solutions to the problems of frequent injections and high cost of traditional recombinant therapy. However, the nucleic acid expression plasmids also have some drawbacks when injected into a mammalian host. The synthetic plasmids of this invention have reduced components, and have been codon optimized to increase efficacy, and reduce adverse reactions in vivo. The introduction of point mutations in to the encoded recombinant proteins was a significant step forward in producing proteins that are more stable in vivo than the wild-type counterparts. Since there is a need in the art to expanded treatments for subjects with a disease by utilizing nucleic acid expression constructs that are delivered into a subject and express stable therapeutic proteins in vivo, the combination of codon optimization of an encoded therapeutic mammalian gene in an optimized plasmid backbone will further enhance the art of plasmid-mediated gene supplementation. Furthermore, the creation of new, improved optimized plasmids allow for better and more efficient production, with lower manufacturing costs and less time per round of production/purification.